all lecture material by austin troy (c) 2003 except where noted lecture 2: introduction to gis part...

All lecture material by Austin Troy (c) 2003 except where noted

Lecture 2:

Introduction to GIS

Part 1. Understanding Spatial Data Structures

Part 2. Legend editing, choropleth mapping and layouts


Introduction to GIS

Part 1. Understanding Spatial Data Structures


Perception, Semantics, and Space• How do we deal with representing semantic

constructions of spatial objects, like “mountain,” “river,” “street,” “city,”

• How about representing more conceptual semantic constructions like “temperature,” “migration pattern,” “traditional homeland,” “habitat,” “geographic range,” etc?

• Answer: we have various data models which use different abstractions of reality

Introduction to GIS


Entities and Fields• There are two general approaches for

representing things in space:– Entities/ Objects: precise location and

dimensions and discrete boundaries (remember, points are abstractions).

– Fields, or phenomena: a Cartesian coordinate system where values vary continuously and smoothly; these values exist everywhere but change over space

Introduction to GIS


Entities and Boundaries• There are two general types of boundaries, bona fide

and fiat (D. Mark, B. Smith, A. Varzi)

• Pure bona fide boundaries represent real discontinuities in the world, like roads, faults, coastlines, power lines, rivers, islands, etc.

• Pure Fiat boundaries are a human cognitive or legal construction, based on a categorization, such as administrative unit, nation state, hemisphere

• Some have elements of both, like soil type areas

Introduction to GIS


Two major data models

• Entity approach roughly corresponds with the vector model

• Field approach roughly corresponds with raster model

• Any geographic phenomenon can be represented with both, but one approach is usually better for a particular circumstance

Introduction to GIS


Raster

• Spatial features modeled with grids, or pixels• Cartesian grid whose cell size is constant• Grids identified by row and column number • Grid cells are usually square in shape • Area of each cell defines the resolution • Raster files store only one attribute, in the form of a

“z” value, or grid code. • Consider the contrary….

Introduction to GIS


• Vector layers either represent:– Points (no dimensions)– Lines, or “arcs” (1 dimension) or– Areas, or “polygons” (2 or 3 dimensions)

• Points are used to define lines and lines are used to scribe polygons

• Each point line or polygon is a “feature,” with its own record and its own attributes

Introduction to GIS

Vector


Raster and Vector representations of the same terrain

Introduction to GIS

Raster: great for surfaces Vector: limited with surfaces


Introduction to GIS

Raster and Vector representations of the same

land use


Introduction to GIS

Raster and Vector representations of the same

land use: closer in


Vector vs. Raster: bounding

Introduction to GIS

Raster: bad with bounding Vector: boundary precision


Introduction to GIS

Vector vs. Raster: Sample pointsCancer rates across space


• In Arc View and Arc GIS, we can covert vector layers to grids, based on an attribute, or grids to vector layers

• The disadvantage of vector to raster is that boundaries can be imprecise because of cell shape• Each time you convert, you introduce more error too

Moving between vector and raster

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WHEN TO USE RASTER OR VECTOR???

Introduction to GIS


• where boundaries are not precise

• that occur everywhere within a frame and can be expressed as continuous numeric values

• where change is gradual across space

• where the attribute of a cell is a function of the attributes of surrounding cells

Raster data analysis is better for representing phenomena:

Introduction to GIS


• Simple file structure

• Simple overlay operations

• Small, uniform unit of analysis

Raster technical advantages :

Introduction to GIS

Raster technical disadvantages :

• Big file size, especially for fine-grained data

• Difficult and error-prone reprojections

• Square pixels are unrealistic


Vector analysis is better :• Where there are definable regions • Where the relative position of objects is important• Where precise boundary definition is needed• Where multiple attributes are being analyzed for a

given spatial object• For modeling of routes and networks• For modeling regions where multiple overlapping

attributes are involved• EG: units with man-made boundaries (cities, zip

codes, blocks), roads, rivers

Introduction to GIS


• Smaller file size (in general)

• More graphically interpretable

• Allows for topology (see further on)

Vector technical advantages :

Introduction to GIS

Vector technical disadvantages :

• Complicated file structure

• Minimum mapping units are inconsistent between overlapping layers


Specific Vector Usages

• All legal and administrative boundaries (zip codes, states, property lines, land ownership)

• Building footprints and 3-D models• Roads• Bedrock geology• Pipelines, power lines, sewer lines• Flight paths and transportation routes• Coastlines

Introduction to GIS


Specific Raster Usages• Terrain modeling where micro-locational variability is

present and matters• Groundwater modeling, where surface flow outside of

channels is important• Representation of slope and aspect• Representations of distance and proximity to features• Spatial representation of probabilities (logit)• Modeling phenomena in nature with continuous spatial

variability and numeric attributes, like soil moisture, depth to bedrock, percent canopy cover, vegetative greenness index, species richness index

Introduction to GIS


• In many cases, though, the choice between raster and vector may not be so clear.

• Often it depends on the application

• The following are some examples where you could go either way:

Tossups

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• Vector-based models used for terrain, including contours and TIN– Problem: creates distinct terrain entities that

distort reality: terraces and triangular facets

• Raster based grids are more commonly used– They are optimal for showing spatial micro-

variation in elevation although still have the problem of being like miniature “steps”

– Lattices deal with this through interpolation

Terrain

Introduction to GIS


Soil

• Soil type: Vector – Soil types are meant to represent discrete and

homogeneous areas and are qualitative. There is no “slight gradation” between soil types like with pH

• Soil pH: raster– pH is numeric, not categorical, and that number may vary

slightly within a single soil type polygon

– If pH were turned into categories, like High, Medium and Low, vector might be better

Introduction to GIS


Weather

• Weather station data: Vector, coded with points• Average precipitation surface: Raster

interpolation of points• Average precipitation contours: vector lines• Both are interpolations, but one may be more

accurate in a given situation• Downside of contours: terrace effect, fewer

intervals, more categorical

Introduction to GIS


Rivers• Most people think of a river as a discretely bounded

entity, hence vector • What about where the river size fluctuates

seasonally, e.g. desert rivers?• Or where the location of the river bed changes

slowly and gradually over the years• Or where the river becomes delta, and the distinction

between “river” and “swamp” becomes fuzzy? • Or where the river has a certain probability of

flowing or being dry at any given location and time

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• Depends on the type of analysis being done• With vector can do network modeling of stream and

river system, but only in the arcs– Vector stream model can take advantage of topologically

enabled analysis tools

• With raster, can do surface flow modeling– More realistic, because when it rains water flows

everywhere, not just in channels, shows accumulation

– Think of every piece of land as mini stream channel

Rivers

Introduction to GIS


• No official administrative boundary for this• Where does one metro area begin and another

end? Look at the New York New Jersey area.• For a precise bounding, say for administrative

purposes, use vector• Can also include “fuzzy boundaries”• To represent a gradual change from one urban

area to another, use raster

Metropolitan Areas

Introduction to GIS


• Vector works well for modeling vegetation stand type where categories are broad, e.g. mixed conifer, deciduous hardwood

• Raster works better where there is micro-locational heterogeneity in species distribution

• Raster also works better for representing ecotones, or edges between two stands

• The more specific and variable the classification, the more likely the raster approach will be needed

Vegetation Mapping

Introduction to GIS


Introduction to GIS

Part 2. Legend editing, choropleth mapping, and layouts


Visual Analysis• The most intuitive form of vector analysis is

visual analysis, where we code features with colors or symbols to deliver information

• Frequently, we code features by an attribute value and let the color or symbol express the attribute value

• Understanding legend editing and map classification is critical to making maps that effectively deliver information

Introduction to GIS


Mapping of Attribute Data

In GIS, each feature can have a number of attributes attached to it (e.g. land parcel>> property ID, assessed value, square footage)

We can map out these attribute values by their corresponding geography

Two basic approaches for classifying the data:

1. Quantities approach

2. Category approach

Introduction to GIS



Quantity approach: applies to numeric attributes that are ordinal (have order to them); this means one values is greater than or less than another; good for continuous data.

Category approach: applies to categorical data, where the categories can have, but don’t need to have, order. If they do have order, the category approach ignore that order

The same layer can have some quantitative and some categorical attributes

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Category approach, example: vegetation type

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Quantity approach, example: population

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Mapping Categories

This is the simplest type of mapping: we are simply assigning a different color or symbol to each feature with a given category value

Examples: vegetation types, land use, soil types, geology types, forest types, party voting maps, land management agency, recategorizations of numeric data (“bad, good, best” or “low, medium, high’). Can you think of any others?

Introduction to GIS


Mapping Categories

To map categories in ArcGIS, we simply double click on the layer in the TOC and, in “layer properties,” click on the “symbology” tab

Generally,we will choose “Categories>> Unique values”

Introduction to GIS

The we choose our values field that contains the attribute and then click the “Add all values” button


Mapping Categories

The symbology in the last slide gives us conservation lands, categorized by type of ownership

Introduction to GIS


Mapping CategoriesOften categories must be aggregated and redefined: this land use

map had over 110 categories that were condensed to 12

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Mapping CategoriesDo do this, we must group the “group values” function in the

symbology properties window

Introduction to GIS

We can then give that grouping a label

In this case 1262, 1263, 1264, 1265, etc. refers to different subcategories of commercial land use

This classification is saved when I save my ArcMap Document


Quantity Mapping

This is more complex, because there are so many ways to map out quantities

Mapping options depends on the feature type:• For points, lines and polygons, we can darken or

lighten the color to express magnitude: this is called graduated color, or color ramping

• For lines and points we can increase symbol size to express greater magnitude: this is called graduated symbol; we can do this because points and lines have fewer than 2 dimensions

Introduction to GIS


Choropleth Mappinga thematic mapping technique that displays a quantitative attribute using ordinal classes applied as uniform symbolism

over a whole areal feature. Sometimes extended to include any thematic map based on symbolism applied to areal objects.-Nick Chrisman

A map that shows numerical data (but not simply "counts") for a group of regions by (i) classifying the data into classes and (ii) shading each class on the map. -Keith Clarke

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Graduated Color

In Arc GIS layer properties>>symbology, we choose Quantities>>graduated color

We then choose a value to representIn this case we choose

median house value

It automatically choosesfive classes for the data

Introduction to GIS


Graduated Color

The resulting map shows high housing value areas with dark colors and low with light

Introduction to GIS


Graduated ColorIn that case we used 5 classes. Changing the number of

classes changes the information delivered; more classes: more info, but harder to see differences

Introduction to GIS

3 classes for median value


Graduated ColorIn that case we used 5 classes. Changing the number of

classes changes the information delivered; more classes: more info, but harder to see differences

Introduction to GIS

15 classes for median value


Graduated ColorThe Classification

Method also affects how the mapped attributes will look. Arc GIS normally defaults to the Jenks, or natural breaks, method

Introduction to GIS

These are the breaks it makes, based on the distribution of the data

largesmall


Graduated ColorNow, here’s an

equal interval approach. Notice how all the breaks are evenly spaced. With a fairly normal distribution of data, this is usually OK

Introduction to GIS


Graduated ColorHere’s what the same

distribution looks like with only 5 equal intervals.

Introduction to GIS


Graduated Color

However, when the distribution is skewed, or there are significant outliers, then equal interval is problematic because most intervals have no data in them. Here’s an example, with number of vacant houses per tract—most have near none, but a very few have a lot

Introduction to GIS


Graduated Color

This map of vacant properties tells us almost nothing, because almost all the records fall into the first class

Introduction to GIS


Graduated Color

Notice how with natural breaks there are now more classes on the left side, where most of the data are

Introduction to GIS


Graduated Color

Introduction to GIS

This map, made with Natural Breaks, is more intelligible


Graduated Color

There is a similar approach to Natural Breaks called Quantile. This method sets class boundaries so each class has equal numbers of observations in it

Introduction to GIS


Graduated Color

This essentially sets the class boundaries so as to maximize the perceived variation in the map, as we see here

Natural Breaks is similar, but does not necessarily result in an equal number of data points in each class; rather it uses Jenks' Goodness of Variance Fit (GVF) statistic

Introduction to GIS


Graduated ColorGraduated color can also be applied to points.

Here are houses display by sales price

Introduction to GIS

Natural breaks Equal interval


Graduated SymbolSince points and lines are not dimensionally realistic, the symbols representing

them can also be graduated. Here the size of the dot represents the house price

Introduction to GIS


Graduated SymbolThe same thing can also be done with lines—for instance, the width of a line feature showing rivers

can be made to represent the flow of that river segment. For many line features, like streets, ArcGIS comes preloaded with symbol palettes that recognize the attribute codes and put the appropriate symbol

Introduction to GIS


Symbol StylesWe can also choose to “match to symbols in a palette” and then apply the

“transportation.style” palette to the CFCC, or roadcategory, attribute in our roads layer

Introduction to GIS

Results in this map

Must click here to match

Choose your style palette here


Symbol StylesOne could also manually create symbol styles for each street type. Clicking on each

symbol in either the TOC or properties windows brings up a manual symbol selector. You can assign a separate one to each category.

Introduction to GIS

Includes many more classes of symbols that are industry standar


Symbol StylesThere are also a huge variety of industry-specific point symbols

that can be either assigned through matching symbols to a predefined style or manually assigning those symbols

Introduction to GIS


Charts displayed geographicallyAttributes for point, line or polygon features can also be

displayed as charts on the map

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NormalizationWith graduated color or symbol, we can also show an attribute normalized by another

attribute or expressed as a percentage of total. Here we have number of vacancies per tract as a percentage of total households. Otherwise we’re only tracking total number.

Introduction to GIS

numerator

denominator


Layouts• You can very simply create a map for layout

in Arc GIS by simply clicking View>>Layout view.

• Layouts are designed to cartographically acceptable, which means they must have the key elements of a printed map, such as scale bars, north arrows, legends and titles. These can be added from the Insert menu

Introduction to GIS


Layouts• Example

layout (from lab 6)

Introduction to GIS

legend

North arrow

Scale bar

title


Layouts• Legends are edited in the Legends property window,

which can be accessed by double clicking the legends. Best way to learn about it is try it out

Introduction to GIS

Legends can show layer name as well as intervals for quantitative data and category names for categorical data


Layouts• You can change names of the layers for the sake of

your layout legend (since most layers have pretty unintuitive names) in the layer properties window

Introduction to GIS


Layouts• In layouts you can have detailed and highly formatted

labeling and annotation. You can use an attribute field to label; this is specified in layer properties

Introduction to GIS


MXD Files • You can save your layout, along with all other

preferences and settings by saving an Arc Map Document (MXD) file. However, this is not saving your data, only the settings, including the layout. If you move the MXD, you must move the layers with it. This is one reason why a geodatabase is easier than multiple shapefiles

• To save, just go to File>>save as

Introduction to GIS


Layer Files • Layer (.lyr) files save all your settings and

preferences for one single file. It is primarily for saving legend settings. So, for instance, if I a layer with 300 land use categories, and I create a legend classification that regroups them into 30 categories, each with a special color or hatching, I can save that as a layer file.

• Once created, opening a layer file will open the data layer with all the preferences saved. You can move the data around without moving the layer file as long as both are on the same system.

Introduction to GIS


Layer Files • This is done in Arc Catalog, by right clicking and

clicking “create layer.” Then I can create the legend preferences in Arc Catalog

Introduction to GIS

Then, double clicking in Arc Catalog will give me the layer properties, which can be changed

all lecture material by austin troy (c) 2003 except where noted lecture 2: introduction to gis part...

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